Humidification systems have long been used to treat patients in need of respiratory assistance. A typical humidification system generally includes a source of gas, a source of water vapor, and a delivery system. In the typical humidification systems, the water vapor is first produced by heating a stationary body of water contained in a humidifying chamber. Then, the water vapor mixes with gases passing through the humidifying chamber, thereby humidifying the gas. However, heating the body of water to a point sufficient to produce water vapor can take a significant amount of time, depending on the amount of water contained in the chamber. In the stationary water humidifier, the humidifying chamber containing the water is arranged so that the water vapor mixes with the gas flowing through the humidifying chamber, thereby humidifying the gas. Furthermore, because the stationary water is heated, when the gas passes over the heated water, the gas is also heated. Thus, a heating step occurs simultaneously with the humidifying step. The humidified gas then proceeds through a respiratory circuit, which directs the humidified gas to a patient. U.S. Pat. No. 5,445,143 discloses such a system.
It is known to implement a stationary water humidifier in a humidifying system having parallel gas flow paths. U.S. Pat. No. 7,146,979 discloses such a system. In particular, U.S. Pat. No. 7,146,979 discloses a humidification system having a valve for splitting a gas into two different paths, wherein one path is humidified while the other is heated by a heater. The system is capable of adjusting the valve to control the relative humidity of the gas being delivered to the patient. However, operating the system in this manner requires implementing sensors for determining relative and absolute humidity of the gas.
Other humidification systems may meter a flow of water to an evaporator. U.S. Pat. No. 6,102,037 describes such a system. The system disclosed in U.S. Pat. No. 6,102,037 provides water vapor with a temperature above 134° C., which heats the respiratory gas. Another humidification system has been disclosed that avoids pumping water and reduces the time required to heat the water by using a capillary system. For example, U.S. Pat. No. 7,694,675 uses a low porosity sintered glass or ceramic to draw water to and through an evaporator tube, where the water is evaporated into a gas.
Regardless of the type of humidifier used, conventional humidifying systems implement a respiratory circuit to provide a flow path from the humidified gas to the patient. Many of the known humidification systems are capable of delivering over 100 Watts to evaporate the water. Therefore, it is preferable to locate the evaporating components away from the patient to simplify system design and minimize patient risk. Furthermore, it would not be comfortable for the patient to have bulky equipment located directly by the patient's face. It is also important to ensure the gas flow being received by the patient arrives at a safe temperature, which is capable of being measured and controlled. By providing a respiratory circuit extending from the humidifier to the patient, the above problems are avoided. However, using a respiratory circuit to deliver the humidified gas creates additional problems.
A significant problem that occurs when using a respiratory circuit to deliver humidified gas is the formation of condensation in the respiratory circuit. Condensation, or “rainout,” occurs due to the temperature gradient existing between the respiratory circuit and the external temperature of the patient's room. The ambient room temperature is generally lower than the temperature of the gases inside the respiratory circuit because the patient's room is usually maintained at a comfortable level for the patient. As humidified gas flow passes within the relatively colder walls of a respiratory circuit, a certain amount of water vapor will condense along the walls of the respiratory circuit. After too much condensation builds up, a practitioner must manually remove the condensation from the respiratory circuit because it is dangerous for a patient to accidentally inhale liquid. The manual removal of condensation requires taking apart the respiratory circuit or replacing the respiratory circuit, which can take a substantial amount of time. Taking down the circuit to remove condensation breaks the continuity of delivering humidified gas to the patient. Furthermore opening the circuit to remove condensation causes a loss of respiratory pressure support and may result in alelactasis or respiratory distress.
Several of the known humidification systems attempt to solve the problem by providing heated elements within the respiratory circuit itself. By selectively heating the heated elements, the operator maintains a temperature of the heated walls to maintain the temperature of the gas above the dewpoint, thereby potentially reducing condensation. Such a system is disclosed in U.S. Pat. No. 7,146,979. However, as discussed in U.S. Pat. No. 6,078,730, in such a system, the temperature is highest close to the wire, but low on the wall across from the heater, thereby allowing condensation to occur. To improve on this system, U.S. Pat. No. 6,078,730 discloses an alternative humidification system that includes a heater wire sitting against or adjacent to an internal wall of a respiratory conduit. Furthermore, DE 4312793 discloses a humidification system having a heater provided in a respiratory circuit. However, these systems require additional heated elements and controls to heat the respiratory circuit walls and the gas to reduce the condensation.
Thus, there is a need in the art for a simple method for reducing condensation in a respiratory circuit.